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COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
Figure 4-65 Effects of Tilt Set Too Low During Descent


CAUTION
Even with a low tilt setting of +2°, it is possible for over-scanning
(♦page 4-40) to occur. Growing thunderstorms that are a threat to
the aircraft may fall below the radar beam and fail to be displayed
on the radar display.
If the tilt is not raised as the aircraft descends, the radar beam will
progressively “dig” deeper into the ground (figure 4-66). The result is
a very colorful display of ground clutter that may fully mask weather
returns (figure 4-67).
In figure 4-66, the aircrew has failed to raise the radar tilt during descent.
At lower altitudes the radar beam is totally immersed in the ground.
Figure 4-66 Result of Not Raising Tilt As Altitude Decreases
If the radar tilt is set to display clutter at the outer edge of the 80 NM
range scale at a cruise altitude of 35,000 feet and the plane then
1st Edition, 1st Revision
18 Sep 03 4-67
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
descends to 5,000 feet without the tilt being adjusted, figure 4-67 shows
the result. Ground clutter completely masks all weather returns.
Figure 4-67 Radar Display with Tilt Setting Too Low
N NOTE
Automatic MultiScan operation optimizes weather returns during a
descent by using one beam to scan ahead of the aircraft and a
second beam at a tilt setting considerably lower than +2° to look
down the aircraft flight path. Because MultiScan is able to look
down into the ground clutter, the radar can detect and protect
against thunderstorms that are growing beneath the aircraft and
would normally fall below the radar beam.
1st Edition, 1st Revision
4-68 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
MID ALTITUDE TILT CONTROL (10,000 – 25,000 FT)
For overland operation at mid altitudes, the best general guideline is
to tilt the antenna until a small amount of ground return appears at the
outer edge of the display. When operating over water ground clutter
may more closely resemble the clutter in figure 4-68, regardless of
which range scale is selected. Should ground clutter be insufficient for
determining the appropriate tilt angle during over water flight, the table
on page 4-72 provides suggested tilt angles for different altitudes.
Figure 4-68 shows the radar set to the 80 NM range scale. Antenna tilt
is adjusted so that ground return is displayed in the outer most range
scale. Note that this picture will look the same when the 40 NM range
scale is selected and clutter is displayed in the outer most range scale.
Figure 4-68 Radar at 80 NM, Mid Altitude
Figure 4-69 shows the radar set to the 160 NM range scale. Antenna
tilt is adjusted so that ground clutter is displayed. Note that due to the
earth’s curvature, it is not possible to get a clearly defined clutter ring in
the outer range scale when the 160 NM range is selected.
1st Edition, 1st Revision
18 Sep 03 4-69
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
Figure 4-69 Radar at 160 NM, Mid Altitude
N NOTE
At mid altitudes, over-scanning of weather targets (♦page 4-40)
begins to become a problem. This is particularly true in high
northern or southern latitudes. Due to the nature of the earth’s
atmosphere, these regions are more likely to have high energy
thunderstorm cells at lower altitudes than equivalent storm systems
closer to the equator.
HIGH ALTITUDE TILT CONTROL (25,000 FT AND ABOVE)
At higher altitudes thunderstorm tops can be all but invisible to radar.
When outside air temperature falls below -40 °C, thunderstorm tops are
formed entirely of ice crystals and reflect very little radar energy (♦page
5-5). Significant down tilt is required to ensure that the radar beam is
picking up the more reflective part of the storm that is at lower altitudes.
Over land ground clutter can be used to determine proper tilt within 160
NM of the aircraft. For longer range targets, special procedures must be
used (♦page 4-79). Within 160 NM, tilting the radar so that some ground
clutter appears in the outer most range scale keeps the antenna pointed
towards the reflective portion of the thunderstorms that are towards the
outer edge of the selected range scale (see figures 4-68 and 4-69).
1st Edition, 1st Revision
4-70 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
Note that while over-scanning (♦page 4-40) of thunderstorms may be a
　
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